Switches actuated by external magnetic fields have been in existence for a number of years. Usually, however, only a single magnetic threshold needs to be crossed to move read contacts from an open to a closed, or a closed to an open position. There have been attempts to make a reliable bistable switch which goes to one state when an external field is applied and then to the other state when a reverse magnetic field is brought into the switch's proximity. A small magnetic chip was mounted on one side of a reed switch to arrive at a degree of self magnetic biasing.
This approach, while effective to a degree, has drawbacks. It is a laborious and time consuming task to mount the chips on the reed switches. The different shapes and problem of aligning the biasing field with the contacts make the bonding of them together tedious. In addition, if the actuating magnetic fields are too strong, the magnetic chip may become depolarized or repolarized and render the switch useless. These problems have been avoided by the present inventor's co-pending U.S. patent application Ser. No. 571,658 entitled "Reliable Magnetically Biased Reed Switch". Yet the co-pending invention does not include the capability for providing a dual threshold switch which is capable of changing its magnetic thresholds of actuation to different levels without physically modifying the apparatus. Thus, there is a continuing need in the state of the art for such a reliable, variable dual threshold switch which inherently possesses a high degree of reliability due to its simplicity of design.